The present invention relates to a device for monitoring electric current in a cable connecting a load and a power source.
Many industrial, commercial and residential environments incorporate large numbers of electrical loads that are widely distributed geographically and often located in sites where access is difficult. Many of these devices are small and draw very limited amounts of current, commonly less than 20 amps and, often, only a fraction of an amp. However, the operation of these loads, for example, fan motors or lights can be important to maintaining a safe environment or the successful completion of a process that may involve costly or hazardous equipment or materials. Monitoring the operation of these devices is complicated by their remoteness from the monitoring location and the remoteness of the controller for the device which is often equally remote from the device and the monitoring location. For example, while security or maintenance personnel may desire to monitor the operation of a building's lights from a central location, the lights of a commercial building are commonly controlled by switches, photo-detectors, or motion sensors located on the floor or in the room where the light is located. Likewise, an operator of an industrial process may desire to monitor the operation of a number of widely distributed devices, for example, the operation of a heater or a fan located in an air duct and controlled by a remotely located thermostatic sensor.
Remote signaling of the operating status of an electrical device is commonly provided by a status indicator comprising a current sensor including a current transformer that is electromagnetically coupled to a cable supplying power to the monitored electrical device or load and a current switch, responsive to the output of the current transformer, to conductively interconnect a pair of output terminals or, alternatively, disconnect the terminals. A relay enabling activation and deactivation of the load by a remotely located controller may be included in the same housing with the current sensor and the current switch. For example, Holce et al., U.S. Pat. No. 6,331,821, incorporated herein by reference, discloses a combination current sensor and relay that includes a pair of switched output terminals and a pair of light emitting diodes (LEDs) that signal whether the monitored circuit is open or a closed. Referring to FIG. 1, the primary winding 58 of a current transformer 60 is connected in series between a load 40 and a source of electric power 28. The primary winding may comprise a power cable that connects the load and the source of power or, as illustrated by Holce et al., may comprise a separate primary winding that is arranged to be connected in series between the load and the power source. The secondary winding 62 of the transformer provides a current or voltage signal representative of the current in the primary winding to a diode D9 which, in conjunction with a capacitor C3, comprises a half-wave voltage rectifier.
The DC output of the rectifier is the input to a precision voltage detector (PVD) 68. The precision voltage detector interconnects its input and its output as long as the input voltage (VDD), the rectifier output voltage, remains above a detection voltage. With the output of the precision voltage detector high, the transistor Q5 conducts and the light emitting diode (LED) D4 is illuminated signaling that current is flowing to the load. The high voltage at the gates of the transistors Q4A, Q4B causes the transistors Q4A and Q4B to conduct, shorting the output terminals 73, 74 which may be conductively connected to an annunciator at a remote monitoring station and/or a controller that controls the operation of the load by exerting a high or low voltage at the coil of a relay 56.
If the rectifier output voltage (VDD) at the input of the precision voltage detector drops below the detection voltage, indicating that current is not flowing to the load, the precision voltage detector interconnects the system ground voltage (VSS) to the output. When voltage at the output of the precision voltage detector is low, the transistor Q5 does not conduct causing the LED D4 to be extinguished and the LED D5 to be illuminated. A low voltage at the gates of the transistors Q4A, Q4B causes non-conductance of the transistors, opening the conductive path between the output terminals to provide a second signal to the monitoring station and/or controller indicating that no current is flowing in the monitored circuit. If the rectifier output voltage (VDD) rises again to a release voltage, the precision voltage detector reconnects the input voltage to its output and the transistors Q4A, Q4B and Q5 resume conducting, shorting the output terminals, illuminating the LED D4 and extinguishing the LED D5.
The precision voltage detector inherently includes hysteresis, that is the detection voltage that triggers the opening of the conductive path between the output terminals is not equal to the release voltage that enables closure of the conductive path between the output terminals. As a result, the current in the monitored circuit that causes the status indicator contacts open will be different than the current that causes the contacts close. Further, when the rectifier voltage approximates the detection voltage, the contacts may open when the power cable current fluctuates but not close following the fluctuation because the rectifier output voltage does not exceed the release voltage of the precision voltage detector. The hysteresis of the precision voltage detector is typically less than five percent of the device's detection voltage but the hysteresis of the status indicator can be substantially greater because the operation of other portions of the current sensor exacerbate the hysteresis of the precision voltage detector.
What is desired, therefore, is a status indicator having increased sensitivity and reduced hysteresis.